As is known, single acting rotary pneumatic actuators are used for the actuation of ball valves, butterfly valves and in general wherever there is the need for generating a rotary movement of less than a full revolution, the return movement of the actuator being obtained by means of return springs which act on the piston of the actuator which causes the forward movement thereof under the action of a fluid under pressure.
One known type of rotary actuator has a linearly reciprocable piston having a toothed rack meshing with a pinion on an actuator shaft which is thus caused to turn through a partial revolution upon linear movement of the piston. Return movement of the piston is achieved by means of a compression spring fitted in the actuator body between the piston and a removable cap of the actuator. In the extended position such a spring has a significant natural length in order to be able to exercise, during the return stroke of the piston, the required force to exert the necessary torsional couple on the actuator shaft, and even when at the end of working stroke, the spring is still held under a certain compression. In some actuators there may be several such springs acting on one or more pistons.
The natural length of the or each spring, when at the end of its working stroke, is always greater than the length of the housing within which it is mounted within the the actuator, so that there is always a preload on the springs. Obviously, the larger the torsional couple to be exerted by the actuator the greater the pre-load it is necessary to give to the spring, and the greater must be its natural length with respect to the length of the housing.
Taking account of the fact that the preload of the springs used in actuators can reach values of several thousands of Kg., it will be immediately seen that their assembly and dismantling requires special equipment and significant safety precautions. In particular, if it is necessary to effect maintenance of a single-acting rotary actuator, or to change the number of return springs to obtain a different torsional couple when remote from the factory or service workshop, one is confronted with a rather complicated job and one which in many cases is dangerous. In fact, usually, at the location of the installation in which the actuator is fitted there is no suitable equipment or specialized personnel for this task, and any attempt to dismantle the actuator merely by removing the screws which connect the head to the body of the actuator can involve significant risks in that the head could be violently thrust by the preload of the spring or springs.
Because of this the maintenance and dismantling of actuators are for safety reasons generally carried out in factories, which means that the actuator has to be sent back to the factory every time for the normal maintenance work, with consequent stopping of the installation on which it is fitted.